| Land degradation of eucalypt plantations after successive rotations limits the development of plantation industry in south China. Therefore, it is important to increase the productivity of the eucalypt plantations by increasing soil nutrient availability. Trails on mixed plantation of Eucalyptus urophylla (clone U6) with Acacia crassicarpa, N fertilizer application to Eucalyptus urophylla (clone U6) were established in Leizhou, Guangdong to determine the relationship between soil N supply and tree growth, and the change of soil nutrient status. At the same time, 4 different sites in Guangdong and Guangxi were selected in order to determine the growth of Eucalyptus urophylla (clone U6) with the application of different fertilizers. The conclusions of the thesis are as follows:1 There was a significant difference among mixing treatments in amount of litterfall. Pure eucalypt treatment had the highest amount of branch litterfall (4.818t/hm2 yr) and the pure acacia treatment had the highest amount of leaf litterfall (17.73 t/hm2 yr). The total amount of litterfall of pure acacia was the highest and pure eucalypt was the smallest among all treatmensts. Concentration of N in acacia leaf litterfall was highest (11.18 g/kg) and branch of eucalypt was lowest (4.44 g/kg) among different components of litterfall. Concentrations of P, K, B in eucalypt leaf litterfall was 0.96 g/kg, 0.31 g/kg, 64.85 mg/kg, respectively, and they were higher than in acacia leaf litterfall. Pure acacia treatment had thehighest total K content in litterfall among all treatments. The treatment with one eucalypt and one acacia tree in the same row had the highest P content in litterfall and the treatment of one row eucalypt with one row acacia had the highest N, B contents in litterfall among all treatments. Intercroping acacia with eucalypt increased litterfall and the best mixing treatments were a eucalypt and an acacia tree in the same row or a row eucalypt with a row acacia.2 The pure acacia forest had the highest concentration of NO3--N, NH4+-N and available K in topsoil, and pure eucalyptus forest had the highest available P in top soil. There was no difference between the 2 treatments of a eucalypt and an acacia tree in the same row and a row eucalypt with a row acacia in available nutrients in top soil. It was proved that mixed plantation increased N availability in top soil in comparison with pure eucalypt plantation.3 Height, diameter and stand volume of trees were different among treatments, but the total stand volume was not significantly different. Comparing to the treatment of a row eucalypt with a row acacia, the stand volume of pure eucalypt was 6.71% lower, but the tree height and diameter were not significant different. Comparing to the treatment of a eucalypt and an acacia tree in the same row, height and stand volume of eucalypt trees in pure eucalypt treatment was 8.7% and 6.99% lower, respectively, but diameter of eucalypt trees was not significantly different. The 2 treatments of a eucalypt and an acacia tree in the same row and a row eucalypt with a row acacia were much better than other mixed treatments.4 Application of N fertilizer to eucalypt plantation remarkably increased NO3--N and NH4+-N in top soil. After half year, concentrations of NO3--N and NH4+-N in top soil were 308.3% and 35.5% lower than those in topsoil beforetree planting without application of N fertilizer. But with application of N fertilizer, they were only 87.3% and 22.3% lower than before planting. The application of N fertilizer increased N supply to tree growth in the early plantation establishment.5 There was a significant effect of N application on biomass of eucalypt trees. The biomass of tree trunk, bark, branch and root in the treatments with N application were much higher than those without N application. Comparing to the treatment without N application, the total biomass of the 2 treatments with 100g and 150g N fertilizer per tree was 144.3%, 95.3% higher, respectively. The lower biomass in the treatment with 1...
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